This Program Project deals with humoral and cellular studies of the IgA system in health and disease. The component projects include: studies of the immune response of normal and IgA deficient individuals to environmental antigens and the IgA effector functions relevant to an efficient disposal of circulating antigens and IgA immune complexes; prognostic factors in IgA renal diseases as related to the molecular forms of IgA, presence of circulating immune complexes, and deposited IgA subclasses and complement components; physicochemical properties of IgA rheumatoid factor and its potential role in the pathogenesis of acute inflammation and the development and function of IgA1 and IgA2 B cells in normal,, immunodeficient and leukemic individuals. The projects are listed as follows: Project I Serum and Secretory IgA Physiology and Immune Complexes. Project II IgA Renal Diseases. Project III Properties of IgA Rheumotoid Factor and Its Complexes. Project IV Role of IgA B Cells in Disease. These projects are interrelated at both conceptual and technical levels, because of the common interest of participating investigators in IgA physiology and immunopathology.